Membranes made from graphene and its chemical derivates show exceptional permeation properties. Defect-free single layer graphene is completely impermeable to all gases and liquids. Due to this intriguing property, graphene-based coating is believed to be a perfect candidate for barrier and protection coating applications. In addition, graphene's mechanical strength, optical transparency, low toxicity and high chemical and thermal stability provide an edge over other barrier materials. However practical applications of graphene based coatings are limited due to the difficulty in growing defect-free large area graphene. It is found that oxidation barrier properties of chemical vapour deposition (CVD) grown graphene with polycrystalline boundaries are actually worse than providing no barrier due to the electrochemical activity of water and oxygen permeated through the defect and grain boundary over an extended period of time. Permeation properties are extremely sensitive to the presence of structural defects and cracks present in the specimens. One possible solution to this problem is to use graphene based multi-layer films.
Difficulty in growing defect free CVD graphene limits the use of CVD graphene for barrier applications. On the other hand despite many efforts to improve the barrier properties of graphene and GO polymer composites, gas permeability is still too high to be used for practical applications.
Graphene oxide (GO) is a chemical derivative of graphene and can be easily and cheaply produced via solution based techniques and can also be easily applied to polymer or other substrates. In dry state it is completely impermeable to all gases and liquids but when exposed to humid condition it acts as a molecular sieve. Even though this property is useful for many applications, its use as a barrier material is limited due to the barrier free water vapour transport. Many practical applications require extremely low water vapour permeation rate <10−6 g/m2/day at 100% relative humidity and at room temperature. One of the possible strategies to overcome this issue is to reduce GO and in doing so decrease its water affinity (see Yoo et al; Graphene and graphene oxide and their uses in barrier polymers; J. Appl. Polym. Sci.; 2014).
Recently efforts have been made to use multilayers of thermally reduced graphene oxide (rGO), and graphene based composites for the use in ultra-barriers for organic electronics, oxidation resistance coating and anti-corrosion coatings. However the thermally reduced graphene oxide membranes (such as those described in CN102040217 and CN 103633333) are extremely fragile and contain many structural defects leading to finite water permeation rate through these films.
Chemical methods have been used to reduce GO. For example, HI and ascorbic acid reduction have been shown to produce reduced graphene oxide that has a lower oxygen content compared to other reducing methods (Pei, S., Zhao, J., Du, J., Ren, W. & Cheng, H.-M. Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids. Carbon 48, 4466-4474, (2010); Zhang, J. et al. Reduction of graphene oxide via L-ascorbic acid. Chem Commun (Camb) 46, 1112-1114, (2010); Moon, I. K., Lee, J., Ruoff, R. S. & Lee, H. Reduced graphene oxide by chemical graphitization. Nature communications 1, 73, (2010); Fernández-Merino, M. J. et al. Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions. The Journal of Physical Chemistry C 114, 6426-6432, (2010)). One problem with chemical reduction techniques is that these also result in structural defects in the rGO. Prior to the present work it was generally accepted that all reduction methods lead to rGO in which the sp2 structure is not restored completely, i.e. rGO with defects in the carbon skeleton still being present (see Yoo et al; Graphene and graphene oxide and their uses in barrier polymers; J. Appl. Polym. Sci.; 2014). The existence of defects in chemically reduced GO is a significant hindrance to the use of this material as an effective barrier material and their origin is not properly understood.
CN101812194 describes barrier materials formed from rGO dispersed substantially homogeneously in a polymer matrix.
It is an aim of certain embodiments of this invention to provide a reduced graphene oxide membrane or composite which is substantially impermeable to gases and water, i.e. which exhibits ultralow permeabilities. It is an aim of certain embodiments to provide a rGO membrane which is less permeable than prior art membranes or composites.
Another aim of certain embodiments of the invention is to provide a substantially impermeable reduced graphene oxide membrane or composite which is less fragile and/or chemically stable than prior art membranes or composites.
An aim of certain embodiments of the present invention is to provide an impermeable reduced graphene oxide membrane or composite which exhibits better adhesion with certain substrates (e.g. metals) than prior art membranes or composites.
It is also an aim of certain embodiments of the invention to provide a substantially impermeable reduced graphene oxide membrane or composite which is flexible. A further aim of certain embodiments of the invention is to provide a convenient and/or economical means for forming a barrier material. It is intended that the barrier can be formed in a relatively simple way. Ideally, it is intended that the barrier can be formed in situ.